Plasma-based light sources, such as laser-produced plasma (LPP) sources can be used to generate soft X-ray, extreme ultraviolet (EUV), and/or vacuum ultraviolet (VUV) light for applications such as defect inspection, photolithography, or metrology. In overview, in these plasma light sources, light having the desired wavelength is emitted by plasma formed from a target material having an appropriate line-emitting or band-emitting element, such as Xenon, Tin, Lithium or others. For example, in an LPP source, a target material is irradiated by an excitation source, such as a pulsed laser beam, in a vacuum chamber to produce plasma.
In one arrangement, the target material can be formed on the surface of a drum. After a pulse irradiates a small area of target material at an irradiation site, the drum, which is rotating and/or axially translating, presents a new area of target material to the irradiation site. Each irradiation produces a crater in the layer of target material. These craters can be refilled with a replenishment system to provide a target material delivery system that can, in theory, present target material to the irradiation site indefinitely.
In some applications, Xenon (e.g., in the form of a layer of Xenon ice coated on the surface of a drum) can offer certain advantages when used as a target material. For example, a Xenon target material irradiated by a 1 μm drive laser having a total laser output in the range of a few kilowatts can be used to produce a relatively bright source of EUV light that is particularly suitable for use in a metrology tool or a mask/pellicle inspection tool. For this application, the laser is typically focused to a focal spot that is less than about 100 μm in diameter.
One of the main remaining challenges in the development of these plasma-based light sources is preventing laser-induced damage of the drum, which is covered by Xenon ice, since Xenon ice is semi-transparent to the drive laser light at 1 μm. Although the laser expands while propagating through the Xenon ice, which is typically about 1 mm thick, the laser power density on the ice surface still remains fairly high and can cause drum surface ablation. The particles that are produced on the surface of the drum can lead to Xenon ice stability degradation over time and also slowly propagate through the ice and lead to contamination of optics and other elements in the vacuum chamber.
With the above in mind, Applicants disclose a plasma-based light source having a target material coated on a cylindrically-symmetric element and corresponding methods of use.